1. Field of the Invention
The invention relates to automatic call transmitters of the dual tone multifrequency type. More particularly the invention is concerned with telephone reporting sets or warning units of this type, however, the principles of the invention are also applicable to repertory dialing devices of the kind referred to.
With the advent of pushbutton dialing, or "Touch Calling," a need has arisen for devices which are capable of automatically dialing desired calling numbers in the dual tone multifrequency (DTMF) mode. In this form of signaling a pair of tones is transmitted over the line, one of which, in response to the depression of a pushbutton of the dial, is chosen from a high group and the other from a low group of frequencies within the voice frequency range. A circuit for such a "Touch Calling" dial is shown and described, for example, in U.S. Pat. No. 3,284,577 to Burns et al, entitled Substation Apparatus Employing a Multifrequency Signaling Arrangement.
Thus, taking the case, for example, of a telephone warning unit--in which, upon the occurrence of an unstandard condition or an emergency, a message identifying the condition and/or the originating location is to be automatically sent to a supervisory station or the like--it becomes desirable to arrange the unit for the transmission of the directory number of the last mentioned station in DTMF form if this is the signaling mode called for by the central office to which the warning unit is connected. To this end such units are equipped with a storage medium, for example of the magnetic variety, in which the desired directory number is initially recorded.
A somewhat similar situation exists in connection with repertory dialers, that is, telephone calling devices with a storage medium containing signaling information which corresponds to the telephone numbers of frequently called subscribers. While in this instance the transmission of an oral message is not needed, these devices, too, must be designed for the automatic transmission of the desired number in the DTMF mode if this is the kind of signaling required by the telephone line in question.
2. Description of the Prior Art
A repertory dialer with the aforementioned capability has been described, for example, in U.S. Pat. No. 3,128,351 to Hohmann Jr. et al. The repertory dialing device according to this patent uses as a storage medium a magnetic drum with a plurality of tracks corresponding in number to the directory number capacity of the repertory dialer. The dialer further has a dual tone multifrequency generator, which in a recording operation, is controlled by ten pushbuttons arranged in two horizontal rows and each of these pushbuttons has associated therewith two individual contacts for controlling the generator to generate a corresponding high voice frequency and a corresponding low voice frequency, respectively. In this manner a series of pushbutton depressions in accordance with a given directory number causes a corresponding series of frequency pairs to be recorded on the selected track of the drum. When the telephone number is to be called, that is during the playback operation, the prerecorded series of frequency pairs is picked up from the rotating drum by the associated magnetic head and, upon amplification, is directly transmitted out over the telephone line.
The technique disclosed in the Hohmann et al patent suffers from the disadvantage that, if the relative motion between the transducer and the recording medium upon playback departs from the motion used during recording, the signals picked up by the transducer have a frequency differing from the one the signals had at the time of recording. These frequency variations lead to the transmission of improper tones over the telephone line. Thus, in order to insure that the transmitted tones are of the proper frequency, a transport mechanism of extreme accuracy with respect to speed control and flutter characteristics would be required. Even though high precision components were used in combination with sophisticated frequency correcting arrangements and equipment for synchronizing recording speed with the read out speed, distortion would still occur and, under certain circumstances, this distortion would result in the transmission of spurious signals. Moreover, even though the above refinements could conceivably reduce distortion to an acceptable level, the additional expense involved would unduly detract from the commercial attractiveness of the device.
A number of alternative solutions have been proposed in all of which mere indicia representative of the dual tones, rather that these tones themselves, are recorded on a storage medium. In U.S. Pat. No. 3,243,517 to Miller et al, for example, a DTMF type pushbutton dial is used and when storage of a directory number on the recording medium--in this case a magnetic drum--is desired, the operation of the pushbutton dial results in the generation, by means of an additional set of pushbutton operated contacts mounted on the dial for this purpose, of a group of d.c. signals corresponding, in terms of a binary type code, to the digit dialed. A four-track recording head is used in conjunction with a cam shaft actuated mechanism for sweeping the head axially of the drum to apply the d.c. signal or signals to the drum and it is the presence or absence of a signal on each of the four parallel tracks that preserves the identity of the dialed digit. On read out the four-track head, again with the aid of the cam shaft actuated sweeping mechanism, simultaneously senses the presence or absence of the recorded d.c. signal indicia on each successive group of four-track parallel recordings. This information is then converted into the conventional two-out-of-seven DTMF code and is used to initiate the generation, by the dial oscillator, of conventional dual tone multifrequency tones for application to the telephone line.
Another of these alternative solutions is exemplified by the repertory dialer disclosed in Schenk U.S. Pat. No. 3,430,004. In this arrangement a single sequence of binary coded d.c. pulses is serially recorded, for each digit, on a magnetic belt under the control of a pushbutton set or keyboard designed to provide d.c. signals in a two-out-of-seven code. Complicated electronic circuitry is used to derive the sequence of binary coded d.c. pulses from these signals. This includes a row memory and a column memory in which the row and column signals received from the dial are converted to two 2-bit binary signals respectively identifying the row and column of the actuated pushbutton; a row comparator and a column comparator; a row code generator and a column code generator; a pulse generator; and a muting gate. In playback a special converter is required for converting the series of recorded, binary coded d.c. pulses to DTMF tones for transmission over the line. This special converter comprises, in addition to a dual tone multi-frequency generator proper and the aforementioned row and column code generators, a code detector as well as a row frequency selector and a column frequency selector.
In the repertory dialer shown in Fischer et al U.S. Pat. No. 3,385,934 the indicia representative of each dual tone is in the form of time-division modulated bursts of a single tone. More particularly the arrangement proposed in the Fischer et al patent uses a special pushbutton dial--again having an individual contact per pushbutton--in connection with an RC gating circuit to cause, by means of a single frequency oscillator, oscillatory bursts of varying time duration to be recorded on the recording medium. On playback a multi-track printed circuit commutator is used for translating the time division recording to a conventional two-out-of-seven-code which is used to control the operation of a DTMF oscillator.
From the foregoing description of the last-mentioned three patents it will be apparent that each of these alternative solutions requires equipment, either electronic or mechanical or both, of considerable complexity which greatly adds to the cost of manufacturing and maintaining the device and tends to detract from its reliability.